Palaeomagnetism and Potassium-Argon Ages of Volcanic Rocks of Ngorongoro Caldera, Tanzania

Abstract

Summary Natural remanent magnetization (NRM) has been measured in 21 lava flows and 1 tuff in the south-west wall of Ngorongoro caldera, Tanzania. The lowest three lavas are normally magnetized, the next two have intermediate directions, and the remainder are reversed; potassium-argon dating places the reversal at 2.45 My, corresponding to the Gauss Matuyama epoch boundary. Alternating-field demagnetization was effective in removing the ubiquitous lightning-produced secondary magnetizations, but partial thermal demagnetization was not. Curie temperatures of the lavas fell into two groups: 250°-350°C and 570°-585°C, probably corresponding to unoxidized and oxidized titanomagnetites respectively. NRM directions in the lavas occurred in several distinct and stratigraphically continuous groups, suggesting intermittent eruption of groups of lavas. No correspondence was found between NRM groups and other parameters, such as Curie temperatures. The dispersion of mean NRM directions is large in comparison with late Quaternary lavas from low latitudes in the Pacific basin, but probably is not large relative to the proportion of the geomagnetic field in Africa that is non-dipolar. The mean palaeomagnetic pole is offset 10.5° from the geographic pole, but because of the large dispersion, the difference is not considered significant.